Introduction
Lisa Ford requested a short article on Cavitation but I decided to
expand on the subject and offer it more broadly, starting with something
just for Fila folk. She had dogs evaluated by PennHIP and got a fantastic
DI (distraction index) reading of 0.30 on one side, but her other hip was
unreadable due to cavitation. She asks: "What exactly does this
mean? I
had a bitch that got a .30/.33 and then was rated with the OFA as 'Fair'.
It is my plan to post this article to my webpage as an
explanation/validation of my dog's lack of a left hip score."
First, let
me assure Lisa that she should celebrate those wonderfully low readings;
despite what OFA said, the odds of that bitch ever getting degenerative
joint disease (DJD) are extremely low... the bitch has hips like a
performance Sighthound! I would recommend that she find out if the bitch
can be done again free for the purpose of PennHIP database increase, just
as a matter of additional proof to any doubting Thomases. But if not, I
would be very happy to own dogs with such an evaluation. Especially in
this high-HD-risk breed.

Every so often, as the author of the book, "Canine Hip Dysplasia",
I am
asked if there is anything new in the field of orthopedic disorders. It
always reminds me of the eccentric owner-editor of a Penns Grove NJ
newspaper in the 1950s who was criticized often for putting "old news"
into his paper; his reply was always the same: "If they haven't heard
it
yet, it's still news!" Sometimes I relate that anecdote as I look over
a
seminar audience obviously composed in part of people who "haven't
heard
it before". But there really are new things going on all the
time, which
is why I keep getting lecture assignments and why I am bringing out a new
edition of the book soon. They may not all be quantum leaps, but people
who went to my slide shows a couple years ago will find it well worth
while to go again now.

PennHIP
There are a number of terms in the new knowledge of HD and the
continuing battle to improve hips that will be unfamiliar to many of you.
Others may have heard them on e-mail "lists", the speedy news
and gossip
line of the late '90s. The most well-known of recent years is "PennHIP"
This is a clever acronym for "University of Pennsylvania Hip Improvement
Program", and was developed by the surgery and engineering professionals
at the Vet School in Philadelphia, most notably Drs. Smith and Biery. In
the early 1980s, older concepts were refined and perfected so that laxity
(looseness of the ball-and-socket joint in the hip) could be given a
mathematically-precise quantitative value, something never really done
before. In the succeeding decade and more, steady investigation into this
technique has resulted in numerous studies reported in the veterinary
journals, and replication of results at various schools. The dog fancier
must first understand that there is a distinction drawn between true
diagnosis and prediction of HD. Diagnosis is generally defined as the
presence of abnormal changes such as shape remodeling and/or "deposits"
and arthritis. Laxity, used as a definitive diagnosis of HD by OFA, is
perhaps really a prediction that the degenerative joint disease (DJD)
will sooner or later occur. The PennHIP people prefer to keep the
distinction, saying that their measurement of laxity is actually a very
valid predictor of relative risk for later DJD. In my experience,
degeneration happens eventually and so regularly that for practical
purposes, the dog owner can safely say that laxity is dysplasia. The
science is sound, the data is voluminous, the evidence is clear: it is
not worth it, in most cases, to breed dogs with excess laxity.

Distraction Index and Covert Laxity
The big difference that enables the PennHIP method to find what I have
coined "covert laxity" which may not be apparent in the OFA view,
is that
the latter is an extended-leg, unnatural position that actually winds up
the soft tissues (ligaments, tendons, muscles) in and surrounding the
joint capsule. This is like putting two sticks into an elastic band and
turning them in opposite directions: a few turns and the band is very
tight and the sticks not loose. The extended leg view, adopted a long
time ago by the AVMA as a standard procedure, tightens the capsule in a
similar way, forcing the ball tighter into the socket than it would
normally be if the dog were in a more neutral position, such as standing
with femurs vertical. PennHIP starts with an AVMA view to see any DJD,
then this loosest, most neutral position, only the dog is anesthetized
and upside-down on the table instead of standing and awake. Now, if the
joints were so horribly loose that the balls nearly fall out of the
sockets in that position, you would not get a meaningful reading or
comparison between dogs. Therefore, this variable is removed by pressing
the femoral heads into the sockets so that all dogs' hip joints look
pretty much alike in this X-ray picture. The third picture is taken when
the heads are distracted (levered) out of the acetabular sockets as far
as they will go. A specially made instrument is used to separate
(distract) the femurs and of course their heads, and the shot is made.
One dog's hips will be looser than another's, and by a specific,
measurable amount. Because some heads are larger than others, a simple
formula is used that divides the distance distracted by the radius of the
ball; this resultant number is called the distraction index or DI. The
less the difference between fully seated and fully distracted, the lower
this number and the lower the risk that the dog will get as many (if any)
of those degenerative signs of HD. After 20,000 dogs, the theory has been
unquestionably proven. There seems to be a threshold DI of 0.3
(equivalent to a maximum distraction of 30% out of and 70% in the
socket), below which the risk for all practical purposes is zero. Look
for "DI" in more breeders' future ads.

Cavitation
A word very few will come across when using the advanced PennHIP
evaluation is "cavitation". The space between the acetabulum and
the
femoral head (the socket and the ball) has a small amount of lubricating
liquid called synovial fluid that performs a function similar to that of
the grease in your car's ball joints. However, it also apparently has
another critical function, and that is related to the fact that it is
permanently sealed in, and if you were to try pulling the ball out of its
socket, there would be a vacuum-like effect. The harder you pull, the
harder it resists. Of course, if you puncture that joint capsule, you
would let in some air or fluid, and the "vacuum" would be lost.
Likewise,
if the dog had a genetic defect that allowed production of excess
synovial fluid, the negative pressure would not be as significant, and
you'd be able to pull the ball further out before resistance built up. In
order to get a good reading of the difference between compression and
distraction views, and measure the displacement to give that index, the
vet has to exert a considerable force, up to the point where even more
force no longer produces any more displacement. The limit to which he or
she can pry the bones apart illustrates a principle of physics and
mechanics known as Hook's Law. A spring or elastic material can be
stretched with relative ease up to the point of its elastic limit, and
will stretch no further, no matter if you add one ounce to the weight or
100 pounds. Of course, if you put a ton or so on the steel cable or
spring, you might exceed the elastic limit and the steel will fail, and
not regain its length the way it did when you stayed within the limit and
relaxed the tension.
In distracting a dog's hip joint, a slightly similar thing happens, but
you do not go past the yield point the way you can with an enormous
weight on the steel test piece. Instead, something else "gives",
a
temporary and harmless phenomenon that is quite rare (about 4%) in the
distraction procedure: voids appear in the synovial fluid and give a
sudden decrease of pressure in the articulation area. Nitrogen and/or
other normally-dissolved gases in the fluid are subjected to enough
suction force that they vaporize, and can show up as radiolucent areas on
the radiograph. When it happens, it is almost always only one hip that is
involved. Within minutes to an hour, they re-dissolve and the dog can be
re-tested. If it takes longer than the vet wants the dog to be under
anesthesia, the PennHIP evaluators in Philadelphia will just take the DI
reading of the other hip. In the even less frequent instance of a vet
causing cavitation in both sockets, that film is rejected or the vet has
already realized what he did, and repeats that part a little later.
A
similar pop or snap happens when one cracks his knuckles, and it takes a
while before it can be done again. I've been annoying family members that
way for fifty years with no damage, and there is likewise no damage to
the dog when cavitation occurs.

Breed Value
Another new term that is just now catching on in the dog world in one
respect is "breed value" or, as the Germans call it, "Zuchtwert".
The way
the magazines and e-chatter play it up, you might think it is new, but it
has been at the core of food animal breeding for a long time. Moreover,
the geneticist in charge of the Seeing Eye breeding program has employed
it for more that twenty years. So why is "ZW" suddenly in the
news and
being implemented by breed clubs? Why had they not used it long ago?
Good question. Some people, especially some dog people on some countries,
are notoriously slow to put good ideas into practice. They are all
protecting their own sacred oxen from being gored. In the U.S., for
example, we showed as far back as the early 1970s that the GSD Club of
America had the responsibility of requiring hip certification for the
dogs they officially promoted as their best breeding stock (the
"Selects"). However, the leaders have consistently turned down
the
membership when the proposal was brought up again and again. Perhaps
there was something of a head-in-the-sand attitude; perhaps there were
just not enough geneticists involved with dogs and dog clubs. In any
case, a big boost was recently given the concept when the German Shepherd
Dog Club in Germany (SV) adopted ZW as a means of informing its
membership of the potentially faster progress that could be had. The
practice of simply excluding the severely dysplastic dogs from breeding
and registering had reached a plateau, and breeders obviously needed
another rung to step on. Smaller clubs such as the Bernese Mt. Dog and
Jagdterrier associations in Germany and elsewhere in Europe had shown
real progress in reducing HD incidence, so the SV decided to use it, too.

Dogs are given rather arbitrary ZW or BV index numbers reflecting how
far from average their hip production is or has potential to be. If a
stud dog is carefully bred only to bitches with exceptional hips or a
good-hip production record, that stud's ZW number falls. Supposedly
everybody starts off with 100 "points", and if the hips of the
progeny
are worse than the parent, his or her number goes up; if they are better,
his/her number goes down, reflecting a better value to the breed. Even if
a dog is not bred, his number can change a little based on what his
parents are or produce, and even what his siblings do in this scenario.
One of the drawbacks is that you could have a litter of dogs whose ZW
numbers all are 100 at birth because that is the average of their parents
(e.g. 105 and 95). Of five dogs that go on to be recorded (registered,
breed-surveyed, and X-rayed in order to be eligible for breeding), one
may "earn" a ZW of 82 by being judiciously bred, while the others
retain
a 99 or 98 without ever having as much as wagged their tails at another
dog. Yet, all may have the same quality hips and the same load of bad vs.
good genes. It just that the one that was bred has had a chance to more
or less "prove" its value, but its littermates have not. Still,
the peer
pressure is on, as the SV has declared that any pairing of dogs must
average 100 or less, or the breeding will not be allowed. So a Sieger
(world specialty show winner) with a ZW of 118 will suddenly find himself
with far fewer bitches to breed to (each must have no more than 82) than
will a lower-rated but excellent dog with a ZW of 85, 90, or 79, for
examples. The breed value concept for dogs is a good one, and though
not
as accurate as it could be, I believe that it may slightly accelerate
progress.

Will They Help?
Do these "new" words add anything to our knowledge of hip dysplasia?
Do
they offer any new tools or at least better handles on old tools, so that
we can dig deeper and get to the bottom of the problem? Can we expect
faster progress with such improved approaches and methods? Yes and no.
There is no doubt that the more you know about the problem, the more
rapid and even possible the improvement. But to take advantage of a tool,
you must use it. So there are two steps to progress: learning and doing.
The PennHIP technique offers so much more knowledge about hip dysplasia
than previous methods, that it is at first surprising that the country's
leading orthopedic foundation had not latched onto it and absorbed it
into its protocols. But there are humans involved with science and
business, and that means potential personality conflicts. This is the
primary if not the only sound reason (or excuse) for not accepting or
copying the new knowledge. What is now available is the ability to
accurately predict at 6 months or even 4 months of age the likelihood of
a dog developing DJD later in life. PennHIP and independent researchers
have found that there is virtually no change in the DI at such a young
age compared to that determined in adulthood. This should not be confused
with the OFA's recent claims that today their own 6-month predictions are
better than 90% accurate. For one thing, the extended-leg view used at
both ages does not give the best picture of laxity; for another, it
doesn't quite ring true that after a quarter-century of maintaining that
accuracy at 12 months was about 70% accurate (in that view), suddenly the
4 or 6 month evaluation is so good. Neither the dogs nor the procedure
has changed, and the vets aren't using different standards now. It
appears more likely to be a reaction to the validity of PennHIP's
early-predictability and accuracy with the new position and technique.
The new technique is here, and those who ignore it will be left behind.
Should breeders replace OFA or OVC with PennHIP? I do not think I would
recommend it at the present time. There is so much history and name
recognition that the OFA number helps breeders indicate to potential
customers that they care, and that their dogs are not severely
dysplastic. The same thing can be said about the German "a"-stamp,
which
is given to any except what the SV (GSD club in Germany) considers
moderately or severely dysplastic. These old schemes are still fairly
useful for a rough comparison with dogs that have not net been evaluated
with the newer method, and are still a bit more recognizable by novices.
The greatest benefit of the OFA reading is that it presents us with the
best position for seeing any calcium deposits or remodeling (DJD). On the
other hand, since the PennHIP scheme includes an identical AVMA
extended-leg view, perhaps the only reason to get an OFA reading is for
advertising. Gradually, as PennHIP attains the widespread public identity
that OFA has earned over more than 30 years, this will change. Or else,
OFA will adopt the better science of the compression-distraction
procedure - it could happen! For now, the best course of action for
the
breeder who wants to do better than he had been able to before, is to
have the PennHIP vet put an extra film into the cassette when taking the
first radiograph, the one of the extended-leg position, and send it to
OFA for evaluation. This way, breeders can compare DI and OFA rating for
themselves and make up their own minds as to what the future procedures
should be.

Percentiles, Medians, Means, and Decisions
Should breeders add the newer knowledge PennHIP offers to their
decision-making processes? Without a doubt! To purposefully ignore the
chain saw sitting on the tailgate of your pick-up truck while you try to
cut down a tree with a small hatchet could reasonably be termed
foolishness, and to ignore the newer and better tools in your approach to
progress in decreasing genetic disease is in the same category. After you
have learned enough of the information, the action steps are necessary,
as I mentioned a couple of paragraphs earlier. PennHIP does not make
breeding recommendations, only evaluations; it leave the counseling up to
your veterinarian and peers. If you get the PennHIP report and it
says
your dog is in the 50th percentile, that means that 50% of its breed have
tighter hips than it does, and 50% are looser. In the 80th percentile
means that your dog's hips are tighter than about 80% of the dogs in its
breed, and as you have guessed by now, the important thing you learn from
both OFA and PennHIP is that "tighter is better". You are perfectly
able
to decide which dog is better for breeding from the standpoint of hip
joint quality.
Another reading you will find on your report is how your dog's hips
compare to the median DI (middle of the range) of the breed. Naturally,
you will want to breed only those dogs that are in the higher percentiles
and have a DI lower than the mean (since the index increases in
proportion to the laxity, smaller is tighter). In addition, you will be
informed by your vet what the current mean (average) DI is for your
breed; some of you may have breeds with small numbers of dogs evaluated
or a rigorous program stimulated by your club's code of ethics, in which
cases the DI may have changed since your fellow Hungarian Trufflehound
breeder's dog was tested a couple of years ago. The best progress will be
seen by selecting the lowest numbers, or at least those below the mean
and breeding to a dog with a lower DI than your own dog's. For all
practical purposes, a DI of 0.3 will guarantee your dog will not develop
HD (DJD). The higher the number, the greater the risk.
If, as is certainly indicated, the DI gives a better picture of future
hip quality in your dog, then deductive reasoning would lead you to think
of it as a reasonably accurate indicator of the genotype of your dog.
That means a better idea of the proportion of bad hip genes to good hip
genes, which in turn means relatively how many bad genes are likely to be
transmitted to the next generation Now that, dear friends, is really
revolutionary. The poor progress we have seen in modern times, with
ratings by BVA, OFA, SV, ADRK, OVC, and other breed and veterinary
organizations is a direct result of their inability to indicate those
hidden genes. A dog that has a good picture in the extended-leg view yet
still produces an unacceptably high number of dysplastic offspring has
too many of those hidden genes. Since OFA would be the first to tell you
of the link between laxity and HD (they actually use that as a
definition), the only reason for the poor progress is that hidden, covert
laxity I earlier mentioned. Therefore, using the logical process of
thought, if PennHIP shows more of this laxity than shows up in the AVMA
type view, it in effect shows us the effects of more "hip genes".
Since
mapping the dog's genome (at least finding markers for enough of the
perpetrators) is decades away, the DI evaluation as promoted by PennHIP
is by far the best tool in our tool chest.

(copyright, Fred Lanting; permission to re-use required)
Fred Lanting is the author of "Canine Hip Dysplasia" and presents
seminars around the world on this topic and gait-&-structure, sometimes
in conjunction with judging assignments, other times as stand-alone
events.